Runtastic had apps that used to do this, where you held the phone and it counted the reps, push-ups, sit-ups etc. There were apps for each activity, which gave you reps, a count, and a timer, along with cooldown, with voice prompts.

Then Adidas bought them out and closed the individual apps and released a replacement for the main Runtastic app.

There were numerous threads on reddit asking for similar replacements, and I don't think there has been one. So I gave up looking.